chapter – 2 - information and library network...

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CHAPTER – 2

LITERATURE SURVEY

2.1 Compact pulsed power

Compact pulsed power technologies are characterized by concentration of energy both in time

and size to generate pulses of high intensity. Nowadays the pulsed power field is driven by size,

weight, and volume constraints and certain applications require technology that can be deployed

in smaller volume under stressful environments. In both the military and commercial

applications, there is an overwhelming need to provide more and more capability in ever smaller

and lighter packages [20]. The need for higher energy density, power density and efficiency is

driving further progress in the field. The pulse conditioning system is an important part of pulsed

power system, its general functions are the storage, switching, and scaling (Figure 2.01) [21].

Pulse Conditioning System

Figure 2.01 Basic pulsed power supply architecture

In order to minimize the weight and volume of the system, two types of system architectures are

routinely considered. The first architecture is one that stores all the energy required for multiple

pulses and then uses an opening and closing switch to transfer the energy to the load (Figure

2.02). This approach is the simplest in operation, but requires a much larger volume for the

energy storage.

Opening & Closing Switch

Figure 2.02 Multiple pulse store architecture

Prime Power

High Voltage Power Supply

Shape Load Store Scale

Prime Power Source

Charging System

Multiple Pulse Store

Load

43

The second architecture is a single pulse storage system with repetitive charge system (Figure

2.03).

Charge switch Closing switch

Figure 2.03 Single pulse storage with recharge The single pulse system will provide the minimum energy storage volume, but requires increased

complexity. The minimum system volume and thus weight will be one in which all the

operations required to store, scale, and shape the load pulse are performed in the same volume as

illustrated in Figure 2.04.

Figure 2.04 Ideal compact pulse power system

The pulsed power has expanded its capabilities in the later half of 20th centaury with the research

and development in improved materials and electrical components with high energy and power

densities. Multidisciplinary university research initiative program was also started in United

States during the starting of 21st century to study the fundamental issues for development of

compact pulsed power systems [3]. The recent advancements in high energy density components

(capacitors, cores etc), and new solid-state devices have enabled pulsed power systems to

undergo reduction in size and weight [22]. Modern electronic products depend on pulsed power

to convert ac or battery power, as the pulsed converter become smaller, and occupy an ever

smaller portion of the product volume. Computers that were once giants have now shrunk to

pocket-size. Our appetite for small, lightweight, yet powerful devices extends well beyond the

domestic purpose to include defence, industry, medicine, and research. The demand is increasing

Prime Power High Voltage Power Supply

Shape Scale Store

Load

Prime Power Source

Charging System

Multiple Pulse Store

Load

44

for pulsed power to serve ever smaller applications with reliable, efficient, lightweight power

sources and the response has been a sustained trend toward compact pulsed power technologies.

The compact pulsed power research is more application specific nature than an organized effort

[23]. A compact pulsed power technologist absorbs advanced ideas and components from other

efforts and knit them together into a unique system that may have little to do with the original

component or its intended purpose. For example, a high power solid-state switch, developed for

the traction industry, might be combined with a high energy density capacitor, developed for a

small computer power supply, and the entire circuit powered by a long-life battery developed for

a new generation of cellular phones. While the general demand for compact sources increases,

the case by case development of a new source is typically achieved by raw innovation. Many

historic examples of compact pulsed power are available in the literature. One such paper

illustrates how a commercial switching system becomes smaller with each new generation [24],

while others cite examples of compact components, such as capacitors [25], transformers [26],

and pulse generators [27].

2.2 Research issues in compact pulsed power technology

The development of compact pulsed power technology requires combined efforts at three levels:

efficient and robust devices at component levels, novel circuits and architecture at the system

level, and effective technique to deliver fast pulses at the application levels. On-going research

and development at international level in capacitor technology and dielectric materials has

resulted in significant expansion in several dimensions of the film capacitor operating envelope.

Research in high-voltage film capacitors has produced a self-healing feature that extends

capacitor life under harsh pulsed power uses. The new dielectrics have produced new high

energy density materials that enable capacitors to store more energy in ever smaller packages

[28-29]. The capacitor technology developments

be reached over the last few years. This progress shows no indication of levelling off in the near

future (Figure 2.05).

Figure 2.05 Progress in capacitor energy density versus time [23]

Advancement of capacitor construction

stored energy density, pulse life, and DC life

capacitors used in pulsed power systems

significant improvement in capacitor construction technology. If an internal breakdown should

occur, the metal film is vaporized at the fault site and thereby disconnects the problem area from

the remaining capacitor [31].

Ceramic dielectric materials such as dense titania ceramic and nanocrystalline titanium oxide

developed and studied to make transmission lines with higher energy storage capabilities for

compact pulsed power applications [

45

The capacitor technology developments have enabled much higher energy densities to

few years. This progress shows no indication of levelling off in the near

Progress in capacitor energy density versus time [23]

Advancement of capacitor construction methods and dielectric quality has resulted in improved

stored energy density, pulse life, and DC life. W J Sergeant [30] has reported a review of

capacitors used in pulsed power systems. The use of very thin metalized



such as dense titania ceramic and nanocrystalline titanium oxide


compact pulsed power applications [32-33]. Very low impedance pulse forming

have enabled much higher energy densities to

few years. This progress shows no indication of levelling off in the near

Progress in capacitor energy density versus time [23]

and dielectric quality has resulted in improved

has reported a review of

. The use of very thin metalized film electrodes is a



such as dense titania ceramic and nanocrystalline titanium oxide are


]. Very low impedance pulse forming lines can be

46

made with ceramic materials for low impedance load such as z-pinches. From the literature

survey it was found that the research on high voltage insulation material is one area which can

help in designing very compact systems. This area of research also explores development of high

relative permittivity material. But both of this will lead to material science research for the

development of new materials. The other reliable solution will be to use the best of the material

available in this field and continue development of compact systems. Ceramic materials very

high relative permittivity and could be used for making compact pulsed power system but it is

not much explored for designing pulsed power system due to its piezoelectric properties. The

newer techniques for storing energy could be explored for making compact pulsed power system.

Study of pulsed breakdown in liquid dielectric has received considerable attention in recent years

due to great demands from pulsed power field for the development of liquid dielectric

transmission lines. Liquids dielectric has played a major role as dielectric material in energy

storage, pulse forming, and switching in the development of pulsed power technology. The

switching utilizes liquid breakdown at high fields, thus going beyond just the insulating property

of liquids. However, in energy storage and pulse forming breakdown is detrimental to the proper

functioning of a pulsed power system. It is also obvious that the size of a pulsed power system

has always been a concern, and the effort of making existing systems even more compact will

have to deal with the higher electric fields that come with the inherently smaller distances of a

compact system. Though an abundance of experimental data is available for breakdown in

various insulating liquids, such as water, cyclohexane, and the noble gases, the basic physics of

liquid breakdown has remained unclear. The two classic physical models are the crack

propagation, which was recently transferred from solid breakdown, and the older bubble

mechanism [34]. The main motivation for the development of liquid dielectrics for pulsed power

47

applications is their structural compatibility with high power pulsed system. There are a large

variety of liquids that can be used but for practical usage, mineral oil and water is widely used in

pulsed power systems. Mineral oil has high dielectric strength and small dielectric constant so it

is suitable for making low impedance systems. Water is attractive due to its high dielectric

constant, high dielectric strength and efficient energy storage capability.

The longer duration rectangular pulse has also attracted more and more interest in the

development of pulse power technology. In development of compact pulsed power system, a

general research issue is to improve the system architecture. For example, study the geometry-

driven issues in pulse forming line. A paper discussed the edge effect of the water blumlein for

short pulse widths through computational modelling, and optimized the line design based on

three dimensional simulations [35]. Another paper investigated the effects of folds on dielectric

breakdown and the voltage waveform at the load in a folded blumlein [36]. Alternative

topologies and novel engineering techniques should be investigated and the optimization of the

geometry can be done using any electromagnetic software.

An important current trend also in pulsed power technology is the development of compact

repetitive pulsed power systems, which have many important applications in defence and other

industrial areas. The trend in repetitive-pulse-power system design is toward higher energies,

larger average power levels, and faster pulse-repetition rates. There is a great demand of

repetitive pulsed power system, but there are few technical problem needs to be investigated and

resolved. Repetitive pulsed power generation by compact sources is being developed for

industrial applications. Successful introduction of pulsed power technologies in industries

depends to a great extent on the availability of highly efficient and reliable cost-effective

sources. Compact repetitive pulsed power systems can be improved by enhancing the peak and

48

average power output, increasing the pulse repetition frequency and reducing the equipment size

to meet the demands of the increasing applications [37]. Improvements can be made in pulsed

power system components. The electrical insulation and dielectric breakdown should be

considered as the design criteria of an improved compact repetitive system.

Some issues of fundamental importance are needed to be investigated for compact pulsed power

system developments such as new materials for high dielectric strength and high dielectric

constant, exploring alternate engineering topologies for compact systems, improvements in

switches etc. The research issues are still open on studying and utilizing new materials-

dielectrics, insulators, metals, and interface in the design of components of the compact pulsed

power systems. Fundamental studies on compact pulsed power system development with new

materials and novel engineering techniques are needed to be investigated to reduce the size of the

system. The energy density of pulsed power systems is often limited by the storage capabilities

of the dielectric subsystem. Advances in pulsed power switches, capacitors, pulse forming line

are necessary to develop compact, reliable electrical power on directed energy systems as well as

space platforms for military applications. Research is also underway by to gauge the future of

compact pulsed power by investigating the fundamental limits of dielectric materials and pulsed

power components.

2.3 Primary energy storage system

The primary energy storage systems generally used are capacitive, inductive, chemical and

inertial. Electrical energy is typically stored in capacitors as electric field and inductors as

magnetic field. It can also be stored as mechanical energy in rotating wheel and flywheel, and in

the form of chemical energy in batteries and explosives. Table 2.01 shows the salient features

and comparison of these energy storage systems.

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Primary energy

storage device

Energy density

(MJ/m3)

Energy /Weight

(J/Kg)

Typical transfer

Time (Seconds)

Capacitors 0.01 – 1 300 – 500 microseconds

Explosives 6000 5 x 106 microseconds

Inductors 3 – 40 102 – 103 micro – milliseconds

Flywheel 400 104 - 105 seconds

Battery 2000 106 hundreds of seconds

Table 2.01 Comparison of energy storage systems

High explosives have the highest energy density and the shortest energy release times, but they

are limited to single-shot operation and require auxiliary equipment to convert their chemical

energy into electrical energy. A battery has a high energy storage density but a low power delivery

capability, and requires both long charge and discharge times. Inertial storage has a high storage

density and a moderate power output capability, and capacitors have the highest electrical

discharge capability but they have a relatively low energy storage density. Releasing this stored

energy over a very short interval to generate peak power by a process called energy compression.

Electrical energy stored in a pulsed power system is released in the form of an intense high

voltage and high power pulse or repetitive pulses. There has been rapid development in electrical

pulsed power systems over the past few decades. Two different means of energy storage utilized

in electrical pulsed power systems are capacitive energy storage (CES) and inductive energy

storage (IES). The circuits of the inductive storage systems are duals of the corresponding

capacitive storage systems (Figure 2.06). The CES has capacitor banks and closing switches

while IES has inductive coils and opening switch. In IES the switch must carry the large coil

current during the storage time, interrupt the current, and then withstand the high voltage

generated by the coil current flowing through the load. The opening switch problem is difficul

enough for single-shot operation in many applications

the practical use of inductive storage in pulsed

obvious when the basic capacitive and

Fig

In the capacitive energy discharge circuit (F

resistor R to voltage V. The time constant for the self

where Rc is the leakage resistance

the charging current can be kept fairly low.

discharged into the load ZL by means of the closing switch S

large compared to the charging current and a capacitive discharge circuit can

current amplifier.

Figure 2.0

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generated by the coil current flowing through the load. The opening switch problem is difficul

shot operation in many applications [38]. There are two major obstacles to

the practical use of inductive storage in pulsed power systems, both of which become

obvious when the basic capacitive and inductive energy discharge circuits ar

Figure 2.06 Comparison of CES and IES

tive energy discharge circuit (Figure 2.07) the capacitor C is ch

voltage V. The time constant for the self- discharge of the capacitor is

is the leakage resistance, may be of the order of tens of minutes. For

the charging current can be kept fairly low. In a practical system the capacitor will typically be

by means of the closing switch Sc. The discharge current is

the charging current and a capacitive discharge circuit can

Figure 2.07 Capacitive energy discharge circuit

generated by the coil current flowing through the load. The opening switch problem is difficult

There are two major obstacles to

, both of which become

inductive energy discharge circuits are compared.

the capacitor C is charged through a

discharge of the capacitor is τc = RcC

, may be of the order of tens of minutes. For such capacitors,

In a practical system the capacitor will typically be

. The discharge current is usually

the charging current and a capacitive discharge circuit can be considered as a

If dWC/ dt is the rate of increase of stored energy in the capacitor and

energy is dissipated in the load, then during charging the capacitor we have

Where R is in series with C and V is the voltage across C.

Suppose the capacitor C is charged by a constant current charger

Then equation 2.01 can be written as

Where T is the charging time of the capacitor and slow charging is best since this gives T

and a low loss in the resistor R while the capacitor is charging.

In the inductive energy storage circuit

the time constant for the L-R circuit is

current source, the switch So and the inductor.

Figure 2.0

For inductive energy storage systems,

have to be charged in relatively short times, a

energy stored in the inductor is transferred to the load by

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is the rate of increase of stored energy in the capacitor and dWR

energy is dissipated in the load, then during charging the capacitor we have

R is in series with C and V is the voltage across C.

Suppose the capacitor C is charged by a constant current charger (It), such that

.01 can be written as,

charging time of the capacitor and slow charging is best since this gives T

and a low loss in the resistor R while the capacitor is charging.

In the inductive energy storage circuit (Figure 2.08), the inductor L is charged to a current I,

R circuit is τL = L/R, where R is the combined series resistance of the

current source, the switch So and the inductor.

Figure 2.08 Inductive discharge circuit

ductive energy storage systems, τL can be in the order of seconds, which means that inductors

have to be charged in relatively short times, and high power primary sources are needed. The

energy stored in the inductor is transferred to the load by means of an opening switch S

R/dt is the rate at which

(2.01)

, such that, V(t) = It/C ,

(2.02)

charging time of the capacitor and slow charging is best since this gives T >> τc

, the inductor L is charged to a current I, and

= L/R, where R is the combined series resistance of the

seconds, which means that inductors

sources are needed. The

means of an opening switch So, which

52

interrupts the current (I) in the charging circuit and a closing switch Sc, which in turn connects to

the load ZL. Due to the rapid decrease of current, a high voltage of magnitude L(dl/dt) is induced

across the opening switch and load, and inductive discharge circuits can, therefore, be

considered as voltage amplifiers.

If dWL/dt is the rate of stored energy in the inductor L and dWR/dt is the rate at which energy is

dissipated in the load the rate of energy in the magnetic field of the inductor, divided by the

energy dissipated in the series resistance in the same time is

(2.03)

Now di/dt = i/T, where T is the charging time and L/R = τL, the inductor time constant, So for a

high charging efficiency, i.e a large (dWL/dt)/(dWR/dt), one must ensure that τL/T is large, which

means charging rapidly with respect to the inductor time constant. It is evident that the two major

technical complexities encountered in inductive energy storage systems are the charging circuit,

because of the necessary fast charging of the inductor, and the opening switch because of its

in built complexities at the currents and voltage involved [39]. Energy storage for pulsed power

systems commonly implies capacitive storage for which the state of the art is relatively well

developed. The components of capacitor banks, including capacitors and switches, are

commercially available and relatively inexpensive. We will be using capacitive energy storage

system for primary energy storage in our project. It is very essential to study and investigate

different type of capacitors and select best suitable capacitor to make a compact system.

2.31 Capacitors for pulsed power

The recent advancements in energy storage capacitor technologies have enabled reduction in size

of pulsed power systems. R. A Cooper et. al. [40] has reported the progress in the reduction of

inductance to 50 nH in energy storage capacitor. Improvement in polymer film quality and

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construction techniques has increased the energy densities [30]. Energy storage capacitors have

been developed over a long time and a lot of them got their names from the material used as

dielectric and the electrode type. The capacitors used for compact pulsed power application are

shown in Figure 2.09. Dielectric is an important component in capacitors to store the electrical

energy. Most of the dielectrics used nowadays are ceramics, plastic films, oxide layer on metal

(aluminium, tantalum, niobium), natural materials (mica, glass, paper, air, vacuum). All of them

store their electrical charge statically within an electric field. The important parameters of these

dielectric used in capacitors are mentioned in Table 2.01

Figure 2.09 Capacitors used in compact pulsed power applications

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Capacitor Type

Dielectric Material Permittivity @ 1 kHz

Max dielectric strength(V/µm)

Min dielectric thickness (µm)

Ceramic Paraelectric 12 – 40 < 100 1

Ceramic Ferroelectric 200 - 14000 < 25 0.5

Film Polypropylene 2.2 650 3.0

Film Polyester 3.3 580 0.7

Film Polyphenylene sulfide 3.0 470 1.2

Film Polyethylene Naphthalate 3.0 500 0.9

Film Polytetrafluroethylene 2.0 250 5.5

Paper Paper 3.5 – 5.5 60 5 – 10

Aluminum Aluminum oxide ( Al2O3) 9.6 710 < 0.01

Tantalum Tantalum pentoxide (Ta2O5) 26 625 < 0.01

Niobium Niobium pentoxide (Nb2O5) 42 455 < 0.01

Mica Mica 5 - 8 118 4 - 50

Table 2.02 Parameters of dielectric used in capacitor [41 - 43]

A ceramic capacitor is a non-polarized capacitor made up of ceramic material and metal in which

the ceramic material acts as the dielectric and the metal as the electrodes. The ceramic material is

composed out of a mixture of finely ground granules paraelectric or ferroelectric materials,

modified by mixing oxides, which are necessary to achieve the capacitor’s desired

characteristics. Paraelectric ceramic capacitor has high stability and low losses for temperature

compensation in resonant circuit application. Ferroelectric ceramic capacitor has high volumetric

efficiency for buffer, by-pass and coupling applications. Ceramic capacitor has high current

limitations so they are not preferred for fast discharge applications.

55

Film capacitors are non polarized capacitors with an insulating plastic film as the dielectric. The

dielectric films are drawn in a special process to an extremely thin thickness, provided with

metallic electrodes and wound into a cylindrical shaped winding. The electrodes of film

capacitors may be metalized aluminium or zinc applied to the surface of the plastic film.

Metalized film capacitors possess self-healing properties that is dielectric breakdowns between

the electrodes are not leading to the destruction of the capacitor. The metalized type of

construction makes it possible to produce wound capacitors with larger capacitance values (up to

100 µF and larger) in smaller cases than within the film/foil construction. Film/foil capacitors or

metal foil capacitors are made of two plastic films as the dielectric each covered with a thin

metal foil, mostly aluminium, as the electrodes. The advantage of this construction is the easy

contractibility of the metal foil electrodes and the excellent current pulse strength. A key

advantage of every film capacitor internal construction is direct contact to the electrodes on both

ends of the winding. This contact keeps all current paths to the entire electrode very short. The

setup behaves like a large number of individual capacitors connected in parallel, thus reducing

the internal ohmic losses and the parasitic inductance. The inherent geometry of film capacitor

structure results in very low ohmic losses and a very low parasitic inductance, which makes them

especially suitable for applications with very high surge currents applications, or for applications

at higher frequencies. The plastic films used as dielectric for film capacitors are Polypropylene,

Polyester, Polyphenylene Sulphide, Polyethylene Naphthalate and Polytetrafluoroethylene or

Teflon .

Kraft paper is impregnated with electrical grade castor oil or similar high dielectric constant fluid

with extended foil plates. It is designed for intermittent duty, high current discharge application.

The advantage of these capacitors is enhanced tolerance to voltage reversal and its disadvantages

56

are lower energy densities and large size. Dry and oil filled constructions are available using a

variety of dielectric materials- polypropylene, kraft and polyester with both foil and metalized

electrodes. In recent times the high energy density achieved from metalized, segmented

metalized paper and segmented metalized polypropylene has allowed designers to benefit from

the higher energy density achieved from this construction.

Electrolytic capacitors have a metallic anode which is covered with an oxidized layer used as

dielectric. The second electrode of electrolytic capacitors is a non solid or solid electrolyte.

Electrolytic capacitors are polarized electrical components. Three families of electrolytic

capacitors used are aluminium electrolytic capacitors with aluminium oxide as dielectric,

tantalum electrolytic capacitors with tantalum pentoxide as dielectric, niobium electrolytic

capacitors with niobium pentoxide as dielectric [44]. The anode of electrolytic capacitors is

highly roughened to extend the surface area which increases the capacitance value. The relatively

high permittivity of the oxide layers gives these capacitors the very high capacitance per unit

volume compared with film or ceramic capacitors. The electrical parameters of the capacitors are

established by the material and composition of the electrolyte, especially by the conductivity.

Three types of electrolytes used are non solid electrolyte which has electrical conductivity ( ~

10 mS/cm), Solid manganese oxide electrolyte is of high quality and forms long time stable

capacitors, its electrical conductivity (~ 100 mS/cm), Solid conductive polymer electrolyte for

capacitors with very low ESR values down to values <10 mΩ, its electrical conductivity (~

10,000 mS/cm). The larger capacitance per unit volume of electrolytic capacitors than other

types makes them valuable in relatively high-current and low-frequency electrical circuits.

The supercapacitor is an electrochemical capacitor whose capacitance value is composed of a

static capacitance stored in electric field with in double layer and pseudo capacitance stored

57

electrochemically with redox reactions and charge transfer between electrode and electrolyte

[45]. The capacitance of double layer capacitor is the charge stored statically in electric fields

within the double-layers on the surfaces of the two electrodes. A double layer is a structure that

appears on the solid surface of an electrode when it is placed into a liquid. The thickness of this

structure acting like a dielectric is on the scale of nanometers so it has the energy density

hundreds of times greater compared with conventional electrolytic capacitors. The pseudo

capacitors store charges through adsorption, reversible redox reactions and intercalation of ions

combined with charge transfer between electrolyte and electrode [46]. Pseudo capacitors can

achieve much higher specific capacitance and energy density than double layer capacitors. The

energy density of a pseudo capacitor can be as much as 100 times higher than in a double-layer

capacitor. In parallel to the research of the pseudocapacitance the conductivity of electrolytes for

double-layer capacitors could be reduced. Low ohmic double layer capacitors for high power

applications were developed. Hybrid capacitors were developed it has new electrode materials

such as Lithium ion to increase the pseudocapacitance.

Capacitors used in pulse power circuits are generally high voltage, fast discharge type designed

to have low inductance and low series resistance. Typically they are charged over a relatively

long period of time and then discharged via a closing switch in microseconds or milliseconds.

The figure of merit used in assessing pulsed power capacitors includes energy density, peak

current rating, inductance and reliable lifetime [47]. The rapid charging capacitor bank power

supplies using solid state devices available in market. These power supplies are prone to failure

in intense electromagnetic environment. Power supplies can be made using simple resistive

charging circuit and its resistance is optimized for the fast charging [48].

2.4 High voltage generator for pulse charging

High voltage generation circuits are either based on

generators offer a common way of generating high voltage impulses that are higher than the

available charging supply voltage.

generators are developed using ceramic capacitors

plastic case capacitors for HPM s

driven circuits are designed to

advantage over Marx generator

driving energy requirement, only one single switch for energy transfer and capacity to run on

high repetition rates. The pulse transformer transfers

capacitor to another capacitor under matched load condition

it is mandatory to have a coupling coefficient between windings of the tr

or 0.153 [51 - 52]. The unloaded waveforms of

shown in Figure 2.10.

Figure 2.10 Unloaded waveforms of

E.A Abramayan et. al. [53

duration transformer based accelerator for intense electron beam generation. P Sarkar et. al. [55]

has also reported the design of half Mega volt transformer for EMP gen

58

for pulse charging

High voltage generation circuits are either based on Marx generator or a pulse transformer. Marx


available charging supply voltage. Fast rise time, low energy and high peak power Marx

using ceramic capacitors for applications in UWB

HPM sources [50]. At lower energy levels (< 5 kJ) p

driven circuits are designed to operate as Tesla or double resonance circuit

generator due to its compact size, low input voltage on primary side, low


The pulse transformer transfers the energy efficiently from

capacitor to another capacitor under matched load condition. For the maximum energy transfer,

it is mandatory to have a coupling coefficient between windings of the transformer

The unloaded waveforms of Tesla transformer for coupling coefficients are

Unloaded waveforms of Tesla transformer output

E.A Abramayan et. al. [53 -54] has reported the design and development of

transformer based accelerator for intense electron beam generation. P Sarkar et. al. [55]

has also reported the design of half Mega volt transformer for EMP generation.

arx generator or a pulse transformer. Marx


high peak power Marx

applications in UWB [49] and using

At lower energy levels (< 5 kJ) pulse transformer

esla or double resonance circuit that has added

due to its compact size, low input voltage on primary side, low


the energy efficiently from energy storage

For the maximum energy transfer,

ansformer as 0.6, 0.385

esla transformer for coupling coefficients are

esla transformer output [51]

54] has reported the design and development of micro second

transformer based accelerator for intense electron beam generation. P Sarkar et. al. [55]

eration.

59

2.5 Intermediate energy storage system

The pulse transformers that are used in the microsecond range cannot be used to transform high-

voltage pulses of nanosecond duration because of higher inductances and capacitances of the

windings that lengthen the pulse rise time and distort the pulse waveform. So, an intermediate

energy storage system is needed to deliver peak power to the loads. An intermediate energy

storage system accumulates electrical energy over a comparatively long time, and then releases

the stored energy in the form of a square pulse of comparatively short duration for various pulsed

power applications. They are also known as pulse forming lines (PFL) or pulse forming networks

(PFN). A PFN consists of a series of high voltage energy storage capacitors and inductors. These

components are interconnected (as a "ladder network") that behaves similarly to a length of

transmission line. The component values of the PFN are optimized to synthesize a pulse output

with minimum flattop ripple. A PFL or PFN is charged by means of a high voltage power source,

and then rapidly discharged into a load via a high voltage switch, such as a sparkgap.

2.51 Pulse forming lines

Pulse forming line is simplest of all pulse conditioning techniques, it starts with slow charging

process and gets tens of nanoseconds output pulses with flat top amplitude. It can be made by off

the shelf available coaxial cable or putting two-three concentric cylinders. Coaxial PFL is the

simplest both in terms of structure and operation. It is simple to construct and easy to operate,

however, the drawback is that the output voltage is half of the charging voltage. A brief

comparison of various types of PFL’s is mentioned in Table 2.03. The electrical energy is stored

in the dielectric medium of the intermediate energy storage system for very short duration

(micro-seconds to milliseconds). Solid and liquid dielectrics are used for pulsed energy storage

in the PFL. The compactness of the system depends up on the dielectric constant of the medium.

60

S.No Characteristics Coaxial PFL Blumlein PFL Strip forming line

1 Characteristic Impedance High Very high Low

2 Size Medium Large Compact

3 Electric Field Low Low High

4 Output Voltage Half of charging Equal to charging Half of charging

5 Operating Voltage Medium High Low

Table 2.03 Comparison of various PFLs

2.511 Pulse forming line using solid dielectric

Solid dielectrics materials have higher breakdown strength compared to liquid and gases

dielectrics. A good solid dielectric should have low dielectric loss, high mechanical strength,

should be free from gaseous inclusions, and moisture, and be resistant to thermal and chemical

deterioration. Studies of the electrical breakdown in solid dielectrics are of extreme importance

in pulsed power applications. When breakdown occurs, the solid dielectric get permanently

damaged, while gases fully and liquids partly recover their dielectric strength after the applied

electric field is removed. The conductivity of solid dielectric is not negligible when the electric

field exceeds more than 100 kV/cm, it increases rapidly with electric field. The conductivity in

solid dielectric at higher fields is due to field emission from electrodes, field aided thermionic

emission and field enhanced ionization of impurities. The breakdown strength can be improved

by ultimately reducing or inhibiting these effects. The main factors that influence the solid

breakdown strength are dielectric type and properties, thermodynamic and mechanical states,

structural defects and impurities, electrode material and surface conditions, applied voltage, gap

geometry, other environmental effects, etc [57 - 58].

61

The static and dynamic nature of high voltage testing is used for the testing of solid dielectric

insulation. M M Morcas et. al. [59] and D B Watson et. al. [60] has reported a brief overview of

the application of statistical methods to establish the insulation strength and life time of solid

dielectrics. The short term solid dielectric breakdown studies is also reported by weibull

statistical analysis [61- 63]. Solid dielectric is generally considered to be non-recoverable in the

event of dielectric breakdown but there are single shot pulsed power applications where its use is

warranted. This can result in increased system capacitance with the possibility of operating at

increased energy levels or reduced system volumes. However solid insulation failure under

pulsed power conditions is not fully understood. Solid dielectric materials have been used as

intermediate energy store for pulse compression using high voltage cables [64]. The

investigation on high dielectric strength with high dielectric constant materials is one area which

can help in designing very compact and sophisticated systems. Ceramic materials has very high

relative permittivity and could be explored for making compact pulsed power system, but it is

not much used in pulsed power system due to its piezoelectric properties [4]. Ceramic mixed

with epoxies are tried and considerable increase in dielectric constant was seen [4]. Ceramic tiles

have been made and it has been possible to charge them up to 60 kV [33].

2.512 Pulse forming line using liquid dielectric

Liquid dielectric is used in high voltage systems to serve the dual purpose of insulation and heat

conduction. The advantage of liquid dielectric is that a breakdown path is self healing and the

temporary failures due to overvoltage are reinsulated quickly by fluid flow to the attached area.

Efforts to understand breakdown mechanisms in a variety of liquid dielectric are continued for

longer time. There is no single theory that has been unanimously accepted for breakdown in

liquid dielectric unlike in gases and solid because of complex and not regular molecular structure

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of liquid. The effect of short- and long-term electric strengths of dielectrics as functions of

various influencing factors is studied and reported [65]. Methods of improving working field

strengths and calculating the static, volt-second and statistical characteristics of the electric

strength of insulation and the insulation service lifetime and reliability are also investigated.

Factors influencing the electric strength includes dielectric material properties, states (pressure,

density, viscosity, temperature, molecular, mechanical stress condition, etc.), electrode material,

electrode surface, contaminations, polarity (for dc and impulse), type (for ac, frequency is also a

factor), duration (for pulses) of the voltage, insulation gap geometry and other environmental

conditions [58]. The increase in hydrostatic pressure will increase the electric strength of liquids

for large electrode areas and for long voltage pulses (>1 µs) [66]. For very pure liquids under

short-term voltage exposure, the main effect of temperature on electric strength is due to the

temperature-dependent density. The electric strength slowly decreases with the increasing

temperature and the decrease in voltage duration weakens this effect [67 - 68]. The dependence

of electric strength on electrode material is due to the variations in the work function for

electrons going from metals to liquids. The electric strength of liquid is mainly affected by the

anode material. Degassing of electrode material increases the electric strength of degassed

liquids for dc and ac voltages by 15-20%. Shielding the electrode surface by ionic layers and

heating the volume of liquid adjacent to micro-tips by high-voltage conduction currents are two

means to increase local electrical conductivity and consequently increase the electric strength of

gaps with liquid insulation. Coaxial pulse forming lines are widely used for pulse compression.

T.H Martin [69] has reported oil based helical pulse forming line having pulse power system

rated 1 MV, 75 Ω, 100 ns with capacitance of 750 pF. The coaxial pulse forming line 60” long,

30” diameter with impedance and transit time of 11 Ω and 15 ns is converted to 1 MV, 260 ns,

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75 Ω. An oil sparkgap has been used as output switch with 0.26 MV/cm breakdown strength.

V.P Singal et. al [70] has also reported the development of blumlein based helical line storage

element for generation of 150 kV, 5 µs pulse. Water helical pulse forming line will also

significantly reduce the size of the system. Water free from mechanical impurities and gas is

used in pulse power system and reduces the dimension of the system. Water has a breakdown

strength ~ 130 kV/cm at pulses with duration of ~ 1 µs and electrode areas of up to tens of square

meters, a large relative permittivity (~ 80) and a small loss tangent in the frequency range of 0 –

1 GHz [71 - 72].

2.6 High voltage switch

The switch is an important component of pulsed power system, which controls the flow of

current in a circuit and also shapes the pulse. The switch used in pulsed power system to transfer

peak power has operating time in range of ~ 1 ns to ~ 10 μs. The switches capable for handling

giga-watt power and having very small jitter time (~ few ns) are frequently needed in pulsed

power applications [39]. The conventional switches are no longer adequate to meet these

requirements so the development of new types of switches becomes necessary. The high power

switches are classified according to the mode of its operation as closing switch and opening

switch. The closing switch is used to make the electrical circuitry and the current increases from

zero to the maximum value with the fall in voltage across the switch. The opening switch is used

to open the electrical circuitry and the current fall from the maximum value to zero with the

increase in the holding voltage of the switch. These switches are generally trigged by external

sources. The rise time of the switch determines its operational limits. The rise time of the switch

is governed by three limits [5].

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- Rate of carrier generation in the insulating medium of uniform field intensity resulting in

critical charge density in a given time.

- Finiteness of propagation time of the electromagnetic wave across the switch

- Circuit model in which the resistive and inductive models determines the rate of rise of

voltage pulse.

Figure 2.11 Switches used in pulsed power system

The switches used in pulsed power systems are shown in Figure (2.11). Sparkgap switch is used

as a closing switch in pulsed power system. It has simple design, low cost, and its outstanding

switching characteristics displays an excellent voltage withstand capacity (around a few MV)

and a high charge transfer capability. Sparkgap switches have fast nanosecond duration closure

time. A gas switch can be considered to be electrically closed when under a high electrical field

stress the insulating gas between the electrodes becomes conducting and a plasma channel

develops. The gap length between the two electrodes should be kept to a minimum to

inductance of the gap. A small gap can however break at an undesirable low voltage, although

this can be avoided by pressurization of the gas medium. The breakdown voltage depends on

both pressure and length of gap.

distance is called paschen curve. The paschen curve for gases is shown in Figure

Figure 2.12 Breakdown voltage and P

The sparkgap switch is still not replaceable when a compact pulsed power

generate high voltage (> 10 kV) and high current (> 1 kA) with

electric pulses. The primary limitation lie

spark gaps are related to electrode erosio

triggering systems [73]. The recovery of the sparkgap between the

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develops. The gap length between the two electrodes should be kept to a minimum to



both pressure and length of gap. The plot of breakdown voltage with product of pressure

distance is called paschen curve. The paschen curve for gases is shown in Figure

Breakdown voltage and Pressure-distance curve for gases

gap switch is still not replaceable when a compact pulsed power

generate high voltage (> 10 kV) and high current (> 1 kA) with fast rising (sub

limitation lies in lifetime and repeatability, other shortcomings with

related to electrode erosion, insulator degradation, high arc inductance, and

recovery of the sparkgap between the first and subsequent pulses is


develops. The gap length between the two electrodes should be kept to a minimum to lower the



with product of pressure–

distance is called paschen curve. The paschen curve for gases is shown in Figure 2.12.

curve for gases

gap switch is still not replaceable when a compact pulsed power system is required to

fast rising (sub-nanosecond)

ther shortcomings with

n, insulator degradation, high arc inductance, and costly

first and subsequent pulses is

66

a very important phenomenon in repetitive pulsed sparkgaps operation [74]. Pressurized

sparkgaps switches with hydrogen gas and pressure up to 120 kg/cm2 has been used for fast

switching up to 100 ps for ultra wide band generation [75]. Moran et. al [76 - 77] describes a

high repetition rate hydrogen sparkgap operating at pressures upto 7 MPa and 0.1 mm to 10 mm

gap. In a triggatron mode, a recovery time of 100 µs has been obtained. Faster recovery times

have been reported when the sparkgap was operated well below the self BDV. Recovery times

lower by an order of magnitude can be obtained by operating the sparkgap below fifty percent of

self BDV. The recovery time of hydrogen is slower at low pressure by an order of magnitude

compared to argon, it is an order of magnitude faster at very high pressure. The recovery of a

mixture of hydrogen and argon falls between the curves for separated gases. Experiments of fast

recovery of hydrogen gas switch were also conducted on a 600 kV, 10 µs, 5 pulse, and 10 kHz

burst mode pulse power system.

Semiconductor switches are used for fast switching fast switching but it is limited by its power

handling capacities. These switches have become popular owing to their compactness, better

service time, efficiency and reliability over other switches in their operational regime.

2.7 Summary

The literature update and research issues in the development of compact pulsed power

technology is discussed. The sub-systems required for the generation of high power pulse are

also discussed. To develop compact pulsed power technology, the combined research efforts are

required at component levels, system level and application level. The new components of the

pulsed power system could be developed using new high energy density materials such as

ceramics. The system level research leads to the investigation of novel architectural technique to

reduce the size of the pulsed power system.

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